1. Field of the Invention
The present invention relates to a subscriber group digital transmitter. More particularly, it relates to a subscriber group digital transmitter, which can recognize a location of a monitoring signal for signaling control when TSI (Time Slot Interchange) function is realized between a plurality of DS1 lines provided with a hybrid signaling system.
2. Related Prior Art
In a hybrid signaling system, a monitoring signal for signaling control (hereinafter referred as to signaling information) of a subscriber and the voice frequency signal (PCM) of the subscriber are transmitted on the same channel (a time slot).
FIG. 8 is a diagram explaining a frame format of DS (Digital Signal) 1. (1) of FIG. 8 shows an unit frame, which comprises time slots TS1 to TS24 required for 24 channels and frame bit "F" provided at the head of the time slots TS1 to TS24.
Each of time slots is composed of 8 bits. One sample value of the voice frequency signal of the subscriber is coded with 8 bits, so that one unit frame is composed of 193 bits. Further, a super frame shown in (2) of FIG. 8 is a multi frame composed by multi-linking the 12 unit frames.
The super frame has twelve F bits. The odd numbered frames of the twelve F bits are called DS1 frames and the even numbered frames are called as signaling frames. These signaling frames express inserting positions of A and B bits showing the subscriber line signaling information based on the North America Standard (for example,TR-008, TR-003).
That is, the signaling information which is a monitoring signal for signaling control is composed of A and B bits. A bits are inserted to LSBs of each time slot of the sixth frame, in which the signaling frames are converted from "0" to "1", and B bits are inserted to LSBs of each time slot of the twelfth frame, in which the signaling frames are converted from "1" to "0".
The A and B bits are inserted to the LSBs of each time slot of the sixth and twelfth frames, respectively, so that the voice frequency signal of the subscriber is coded with 7 bits by using a different coding rule from that in the time slots of other frames (as described above, it is coded with 8 bits in the time slots of other frames).
FIG. 9 is a block diagram showing the conventional structure of the subscriber group digital transmitter 1 which transmits to the plural subscribers 3, the digital signals corresponding the signaling information, and the voice frequency signals of the subscribers obtained by cross-connecting.
The digital signals have the above-described multi-frame structure, which are transmitted via the plural DS1 lines (#1 to #n) from the digital switch 2.
In the conventional subscriber group digital transmitter 1, a signaling information extractor 17 has a function for extracting the subscriber signaling information from the multi-frame signals transmitted via the plural DS1 lines (#1 to #n).
A frame aligner 19 has a function for aligning the frame phases (the phases of time slots) of the multi-frame signal after extracting the subscriber signaling information. The signals the frame phases of which are aligned are converted to serial data in a multiplexing/demultiplexing circuit 11, and inputted to a TSI circuit 10.
In the TSI circuit 10, the time slots for the voice frequency signal of the subscribers are cross-connected per 8 bits. The cross-connected signals are converted to parallel data in the multiplexing/demultiplexing circuit 12 and led to a signaling information inserting circuit 18. In the signaling information inserting circuit 18, the subscriber signaling information which is extracted in the signaling information extractor 17 is inserted to the cross-connected voice frequency signals of the subscribers, again.
Further, signaling information recognizing circuits (#1 to #n) 20 are provided corresponding to the plural subscribers 3.
The above-described conventional subscriber group digital transmitter 1 extracts a signaling information in the foregoing section of the TSI circuit 10, and inserts signaling information extracted again after having cross-connected the time slots in the TSI circuit 10. Accordingly, the structure of the conventional subscriber group digital transmitter 1 generates such a problem that the signaling information is inserted to the position which is not in the signaling frames (the sixth and the twelfth frames).
In the conventional transmitter, the frame phases (the phases of time slots) are aligned in the frame aligner 19, after the signaling information is extracted in the signaling extractor 17, and then the TSI processing is performed. However, the phase of the sixth frame period is not aligned, in the conventional transmitter.
Therefore, it becomes difficult to recognize the position of the signaling information, as it is not sure that the time slots which exist in the signaling frame before exist in the sixth frame after performed the TSI processing in the TSI circuit 10.
As the result, the signaling information is extracted in the foregoing section of the TSI circuit 10 and inserted with the six frames period. However, there is a case where the signaling information is inserted in the position which is not an accurate signaling frame. Then, it brings a problem that VF signal characteristics are deteriorated.
This is because the coded bit number of the voice frequency signal of the signaling frames is different from that of other frames, and it brings a problem that the VF signal characteristics are deteriorated, as the coded bit number of the inserted frames becomes smaller than 8 bits, when the signaling information is inserted to the frames other than the signaling frames.